Known types of steam generators supplied with heated fluid from a nuclear reactor vessel have an outer shell or tube and an inner shell or tube with heat exchanger tubes within the inner shell for circulating the heated fluid within the steam generator. A liquid, such as water, is contained within the shells, which are interconnected for liquid flow, so that the upper level of the liquid is the same both in the inner shell and the outer shell and covers the heat exchanger tubes during operation of the steam generator.
At times, it can be necessary to perform repair operations within the space intermediate the inner and outer shells, e.g. repair of the girth weld in the outer shell which is adjacent the upper level of at least one of said heat exchanger tubes.
It would appear to be a simple matter merely to suspend operation of the steam generator and the reactor and lower the fluid water level in the generator to permit such repairs, but radiation is still omitted from inside the inner shell to which workmen in the space between the inner and outer shells are exposed which is undesirable. In the past, under such conditions, lead sheathing has been applied to the outer surface of the inner shell to reduce such exposure. However, the application of such sheathing is time consuming, resulting in high radiation exposure of the workmen, and is not as effective as keeping the tubes immersed in water within the inner shell.
Since the spaces within the shells are interconnected for water flow, the water level between the shells ordinarily would be the same as the water level within the inner shell so that merely raising, or permitting, the water level within the inner shell to be high enough to provide the desired shielding would be unsatisfactory because the areas on which repairs are to be performed would be immersed in the water.
The problem then arises as to how to maintain a water upper level between the shells which is low enough to permit such repairs while permitting a water level within the inner shell which is higher than the water level between the shells and which will cause said heat exchanger tubes to be immersed.
With steam generators of the type which have been installed in pressurized water, reactor power plants, the steam generator cannot, as a practical matter, be opened up to provide easy access to the space between the shells where repairs are to be made. Access to such space is normally limited to relatively small access openings in the outer shell which are above such space.